As a teenager he was a shipwright's apprentice in Skye, but was called to serve as a spy for the Scottish forces in the second Jacobite rebellion. He served for a year as the drummer boy of King George II of Great Britain, then deserted and took what jobs he could find, continuing to send reports to General Murray by pigeon.
In April 1746, Seamus found a job at an inn called The Hanged Woman, where William Hanover, the Duke of Cumberland, was expected to stop. There Seamus met the time-lost Montgomery Scott, who had been sent into the past by the omnipotent being Weyland. Seamus shielded Scott when he tried to steal food, brought clothing to his hiding place in the barn, and helped him get a job at the inn, all the while hoping to shoot the Duke in his sleep and leave the gun with Scott. However, Seamus was apprehended in his attempt and taken outside to be shot. Scott rode up on horseback, scattering the soldiers and riding off with Seamus.
While fleeing their English pursuers, Seamus and Scott came upon a cabin occupied by a teenage girl named Megan, who had been raped and left for dead by the English soldiers who had hanged her father and brother. The three were trapped inside the cabin by the English officers, but were rescued by a band of Scottish soldiers led by Duncan. The three traveled to Inverness, from where Seamus accompanied Scott on the march to Fort Augustus on the Great Glen. The two men attempted to breach the wall of the fort from the inside with gunpowder. Scott was captured, but Seamus rescued him and succeeded in blowing up the magazine. Seamus then confessed his original intentions toward Scott, who forgave him.
After the calamitous Battle of Culloden, Megan found Seamus and Scott, both wounded. Seamus had a concussion but slowly recovered. Realizing he was dying, Scott made Seamus promise to return to his apprenticeship and make a life for himself and Megan. Seamus had hoped that Scott would be godfather to his children, but in the night Scott crawled out of their hiding place to die and was returned to the 23rd century.
Seamus and Megan MacIntyre had two sons and three daughters. Seamus died in 1803. He was a direct ancestor of Frederick William Jeffries, the designer of the Jeffries tube. (TOS novel: Home Is the Hunter)