Throughout 1975 Lewis continued to work both in television and theatre, appearing in two episodes of the afternoon T.V. drama Rooms and joining the newly-formed Maximus Actors' Arena. This group was a venture set up with an Arts Council grant, initially for one year, and provided lunchtime drama at their theatre in Leicester Square in London. Lewis appeared in two plays during his time with the group, and also had an opportunity to direct, with the play Double, double. But his second T.V. appearance that year, in a situation comedy called The Cuckoo Waltz, would be his really big break. When he first appeared on screen on 27th October as Gavin Rumsey, the rich, handsome young man with an eye for the girls, he became a star. The sitcom, with its 'gentle' humour, was surprisingly popular and Lewis showed a real talent for comedy, but it was mainly his good looks that got him noticed, especially by girls.
The Cuckoo Waltz was so successful that a second series was quickly commissioned, and early 1976 was spent working on this, as well as appearing with Maximus in their final play before the Arts Council grant expired. Lewis then won the rôle of D.J. Leonard Brazil in the Prospect Theatre Company's touring production of City Sugar, a performance which led Prospect to offer him a long-term contract. Music also seemed to be creeping back into his career, with reports of a single to be released with his friend Mr. Peter (now known as Mike McGear) and Cuckoo Waltz co-star Diane Keen, although this never actually materialised.36 He also made his first public film appearance, albeit in a brief, non-speaking rôle. At the same time, a third series of The Cuckoo Waltz went into production and by the end of 1976 Lewis was kept very busy trying to balance a promising stage career with an increasingly successful T.V. one.
In early 1977, the prestigious Prospect Theatre Company were just settling into their new home at the Old Vic Theatre in London, with five plays scheduled for their first season there. Lewis was cast in two of them: one was a production of Hamlet, with Derek Jacobi in the title rôle; the other was War Music, a musical adaption of Homer's Iliad "where I have to look like a Greek warrior and wear one of the smallest costumes I've ever seen in my life!"26 Following its run at the Old Vic in May, this latter production was to embark on a British Council tour of the Mediterranean and Middle East in July. Amidst this, Lewis was already beginning to experience the benefits of small-screen fame, being invited to Portugal to do a fashion shoot for the TV Times magazine.
Now Lewis, having quit The Cuckoo Waltz after the third series, was at a crossroads. A successful career on the stage seemed assured, but T.V. stardom beckoned. His portrayal of Gavin had earned him many fans, predominantly female, and his studio mail was massive, with fans sending him gifts and offers of drinks, marriage, and more. Then he won the rôle of Kilner in an episode of the popular T.V. series The New Avengers and Lewis found himself on the horns of a dilemma. Filming was due to start on 16th May, clashing with the Prospect season. "I realised that it was now vital I made a name for myself, if possible internationally, yet my heart lay in the theatre."4 But he reasoned that being well-known would make him a bigger draw in the theatre, so Lewis took the plunge and opted for celluloid.
Although the part of Kilner was a one-off rôle, he hoped it would be the first step towards bigger things. It was. Lewis's main co-star in this episode was Martin Shaw, and the two reputedly did not get on well together. This was noted by the production team, who remembered it just a few weeks later when they had to replace an actor in their latest venture, a new T.V. series called The Professionals. They needed someone who could portray an abrasive, spirited partnership with the other actor - Martin Shaw – and the first choice for the rôle had been too friendly with Martin: "They would just sit in the car and giggle." As the filming of this new show had already begun, the replacement actor had to be found as quickly as possible, and in record time Lewis was tested, cast and signed up to a four-year contract. "It was a whirlwind," he said. "I had no time to think - I signed on the dotted line."28
The Professionals was an action series about the fictional Government department of CI5, (Criminal Intelligence 5) whose mandate was to tackle serious crime and terrorism in Britain. The show focussed on CI5's controller, George Cowley (played by the late Gordon Jackson) and his two top agents, Bodie (Lewis) and Doyle (Martin Shaw). His character, a former soldier, seemed to Lewis to be rather one-dimensional. "Bodie looked limp and lifeless. No depth. A bit of a show-off but also very weak. Nobody seemed to know much about him. I thought, 'if this guy is meant to be an ex-S.A.S. sergeant, then he had better look it.' So I toughened him up, tried to look as if I could handle myself."22 The active nature of the Rôles required a great deal of stamina and energy from the two younger actors, who throughout the show would perform almost all of their own stunts. They also needed to be able to handle various weapons competently, and from the outset Lewis and Martin were put through vigorous training whenever they weren't on camera. An S.A.S. sergeant was brought in to give advice: Lewis remembered, "Right at the start of the first series, we went off into a wood with him. He kicked us around a bit - terrifying, he was! He taught us how to kill people with our hands. But we could never use that in the show. It's got to be glamorous. It's got to be John Wayne roundhouse fights."
The discord that seemed to exist between Lewis and Martin at the outset was soon forgotten as the two discovered a mutual love of music. Martin played guitar, and confessed that he practised "so that I don’t embarrass myself when Lew and I jam occasionally. Lewis has got a small studio of his own where we put a few things down on tape and have a giggle about them."30 And Lewis remembered, "In between almost every scene we filmed, we were forever sitting in our caravan singing duets of old Everly Brothers hits."57
Filming continued from June right through to December, by which time both actors were exhausted. "It sounds good, doesn't it? Eight months on and four months off - but, my God, you need it!"30 On 30th December, 1977, The Professionals made its screen début. Initial reactions were mixed but the show quickly became a hit with T.V. viewers and Lewis and Martin, already reasonably well-known to the viewing public, became household names. The first series ran until mid-March 1978, by which time the show and its stars were so popular that in April, Bodie and Doyle came joint second in the 'Most Compulsive Male Character' category of the TV Times Top Ten Awards. Both actors were affirmed as T.V.'s latest heart-throbs and London Weekend Television had to start up a Professionals Fan Club to cope with all the letters the show received.
As a 'pin-up' Lewis was in constant demand, particularly with teenage magazines whose readers wanted to know everything about their latest idol, and he was happy to oblige. More importantly, his new star status brought many more offers of film and T.V. appearances, and, ironically, the National Theatre were in touch with him too - "There was me trying all my days to get into the National Theatre, and once I did this, they handed it to me on a plate!"31 But his contract with Mark-1 Productions limited what he could accept: "In many ways it's comforting to know there's work to go back to. But it's also restricting, not being able to tackle any of the offers that seem to be pouring in."30 But he was able to accept some non-acting parts, and in May 1978 he found himself returning to his musical roots by appearing with singer Cilla Black in one of her T.V. extravaganzas. They performed two song-and-dance routines together, which resulted in Lewis being offered his own one-off show, although he wasn’t able to accept this. He did, however, get the opportunity to appear in another musical production later in the year, in Thames Television's Must Wear Tights, again singing and dancing and generally showing his versatility as a performer. Again, it was reported that Lewis would be releasing a single, entitled What Am I Doing Falling In Love With You, although this also failed to materialise.45 And Lewis hadn't forgotten his theatrical ambitions - "Apart from the next series of The Professionals, I would like to go back on the stage. I like classical theatre. I suppose Hamlet would be a rôle I would like to play, primarily because it's one Shakespeare drama I have not appeared in.”20