Simon Vivian, former chief executive of Hanson Building Materials Europe, has been named Mowlem's chief operating officer, following a search that started towards the end of last year.
Building revealed in February that Mowlem had identified Vivian as one of three strong candidates to follow Gains as chief executive.
Gains, however, has announced that he intends to stay on until next May, when he is due to retire. He stands for re-election as chief executive at the company's annual general meeting on 20 May.
Gains could even remain at the helm beyond his set retirement date, although this seems unlikely as head-hunters were instructed to find his eventual successor. A Mowlem spokesperson said: "Although next year is his due retirement date, the board could invite him to stay longer if it so wished."
Vivian left Hanson last summer following a reorganisation of the business. The Mowlem spokesperson said that he had not been in permanent employment since. Vivian is currently on holiday and was unavailable for comment, but it is known that he will start on 1 June.
The board could invite him to stay even longer
Mowlem spokesperson on Sir John Gains’ tenure as CEO
In a statement to the Stock Exchange, Vivian said: "I am delighted to be joining Mowlem at such an exciting time for the group. I see great potential for the business and look forward to the challenge of maintaining the significant growth achieved over recent years."
The 47-year-old will gradually take more responsibility for the day-to-day running of Mowlem, reporting directly to Gains.
Although Vivian got the job, rumour had it that Gains favoured another candidate, Peterhouse chief executive Simon Jackson. However, it is understood that the rest of the board were less convinced.
At one point it was thought that former Mowlem building division boss Steve Bowcott was making the running to succeed Gains. He left Mowlem to join Amec in 2002, but is believed to have wanted a return.