In October 1937 he raised and commanded the Antrim Fortress Royal Engineers at Victoria Barracks, Belfast, one of the first Territorial Army (TA) units in Northern Ireland.

He was appointed Honorary Colonel of the unit's successors, 591 (Antrim) Independent Field Squadron, Royal Engineers, when the TA was reformed after World War II.

The opinion of those former officers who knew him was that his politics were not unlike those of Captain Terence O'Neill and that, had he lived to become Prime Minister of Northern Ireland, the complete history of the province would have been different.

Maynard Sinclair was one of 135 passengers drowned in the sinking of the ferry MV Princess Victoria on 31 January 1953, en route across the North Channel from Stranraer to Larne.

There is also a Major J. M. Sinclair Memorial Pipe Band which was formed shortly after his death and can still be found in competition throughout Northern Ireland and Scotland.