The group, which is controlled by Michael Lagan, is one of Northern Ireland's largest construction businesses.
During the year the number of staff it employed rose from 421 to 510.
A note in the firm's accounts said that "despite challenging market conditions the group has managed to maintain acceptable margins".
The shareholders, who are Mr Lagan and his family, received a £4m dividend.
The accounts show that in 2015 the group sold its share of a roadbuilding joint venture, Amey Lagan Roads Holding.
It also took full control of a water treatment works joint venture, buying out a 50% share held by John Laing Infrastructure.
In April 2015 the group also took control of H&J Martin, another Belfast-based building firm.
Lagan Construction Group has recently been awarded the contract to build the City Quays 2 office block at Belfast Harbour.
Other projects it was involved in during 2015 included the A8 Larne to Belfast road and the second phase of the Ulster University campus in Belfast.
The Belfast-based Lagan Construction Group has made a profit of £4.6m on a £202m turnover in 2015.