Thomas MacLellan, born c. 1605 at Glenshinnoch (present-day Orchardton Castle), Auchencairn, Kirkcudbrightshire, Scotland, was the 2nd Lord Kirkcudbright, a title he held from 1639 his passing in 1647.
On 5 February 1642, he and his wife received a charter for various estates, including Twynholm, Bombie, Loch Fergus, and others, securing his family's lands and wealth once again.
A staunch Presbyterian, MacLellan was a committed Covenanter—one of the faction of Scots who opposed King Charles I’s attempts to impose Anglican reforms on the Church of Scotland.
In 1639, Thomas MacLellan commanded a regiment of 600 Scots at the Battle of Duns Law, where the Covenanter forces, led by Alexander Leslie, defeated the Royalists.
In 1641, MacLellan was implicated in the controversial "Incident of August 1641," a failed plot to strengthen King Charles I’s position in Scotland by eliminating rival noble factions, notably the Marquises of Argyll and Hamilton.
Though MacLellan’s role remains unclear, several accounts suggest that he was involved in secret meetings with other Scottish Lords, discussing ways to secure the King’s authority.
In the Siege of Newcastle and the subsequent military campaigns in Cumbria, MacLellan's regiment remained active, advancing into England to support the Covenanter forces under the Earl of Leven.
MacLellan argued that he held seniority and should assume command of the entire siege operation, but Beecher refused, citing his own orders from Sir Wilfred Lawson.
His regiment also served in various campaigns throughout Scotland and Northern England during the final years of the war, including expeditions to Aberdeen and Banff in October 1645.