Denis McLoughlin was born in Bolton, Lancashire, on 15 April 1918. He studied on a scholarship at Bolton School of Art in 1932, and joined Ward & Copley art studio in Manchester, where he drew illustrations for catalogues and newspaper advertisements until early 1940, when he was drafted into the Royal Artillery. While in service he painted insignia on his regiment's vehicles, murals on regimental buildings, and officers' portraits. This led to work painting covers for paperbacks, and writing and illustrating joke books in collaboration with his brother Colin.
After his discharge in 1946 McLoughlin signed a three year book cover contract with T. V. Boardman. In 1948 Boardman started publishing comics, reprinting American titles as well as commissioning original Swift Morgan, Roy Carson and Buffalo Bill stories from the McLoughlin brothers - Colin writing, Denis drawing. They produced the Buffalo Bill Wild West Annual (1949-61) and other annuals for Boardman, and Denis continued to illustrate book covers until the company closed in 1967. He also produced annuals based on western TV shows for other publishers through the 1960s.
He started working for IPC in 1967, drawing "Saber" and "Big Hit Swift" for Tiger. He drew "Fury's Family" for Lion in 1971, then took a couple of years out of comics to finish his book, Wild and Woolly, an encyclopedia of the old West published by Doubleday in 1974, but when he returned to IPC there was no work available for him. He turned to DC Thomson, drawing for The Wizard, The Victor, Buddy, The Crunch, Bullet and Scoop between 1974 and 1983. He then drew two western features for the 1983 Look and Learn annual, and began drawing for Commando, for which he drew roughly an issue a month for the next nineteen years.
On 22 April 2002, a week after his 84th birthday, McLoughlin took his own life with a Colt revolver that had been part of his reference collection.