Lagan was built to the RN's specifications as a Group II River-class frigate.

The ships were designed by naval engineer William Reed, of Smith's Dock Company of South Bank-on-Tees, to have the endurance and anti-submarine capabilities of the Black Swan-class sloops, while being quick and cheap to build in civil dockyards using the machinery (e.g. reciprocating steam engines instead of turbines) and construction techniques pioneered in the building of the Flower-class corvettes.

After commissioning in December 1942, Lagan served in convoy escort missions and participated in anti-submarine warfare exercises off Lough Foyle.

At 04:57 on 20 September 1943, a GNAT torpedo struck the stern of Lagan, causing critical damage to the ship and 29 dead.

She was towed by the tugboat HMS Destiny to Mersey, arriving on 24 September, where Lagan was declared a constructive total loss.