Spencer is a sleek engine privately owned by Duke and Duchess of Boxford's private engine.
During his first visit to Sodor, Spencer took the Duke and Duchess of Boxford on a trip around Sodor. He ignored Gordon's warnings about taking on water and thus ran out - Spencer attributed the incident to a leaky tank.
Upon his next visit, Spencer challenged Edward to a race and maintained a lead for the majority of the race, but vainly fell asleep while the Duke and Duchess stopped to take photos and lost to Edward, after Edward got onto the summer-house branch first.
Spencer has since developed a rivalry with Thomas, with the latter often trying to outshine Spencer - largely without success.
Spencer returned to Sodor to help build the new summer house for the Duke and Duchess of Boxford, much to the annoyance of all the engines. First, he insulted the other steam engines - especially Thomas by calling him a "toy tank engine", which as a result they had a competition of strength. However, Spencer neglected to build the new summer house when he became suspicious about Thomas, thinking he was being a tricky tank engine and ended up getting slate down his funnel at Anopha Quarry. When Spencer discovered Percy's mail coaches (thinking that Thomas was hiding them from Percy), he got into trouble with the Fat Controller, who thought that Spencer was hiding the mail coaches all the time. Afterwards, Spencer lost his pride when his owners turn up for their holiday. He then gets stuck in the mud when he chases Thomas to Knapford. After being rescued by Hiro, Spencer apologises to Thomas and Hiro and they all work to finish the summer house.
Sir Robert Norramby borrowed Spencer as his private engine during the restoration of Ulfstead Castle. He transported the Earl around, and, much to his embarrassment, had to pull some stones for the job. During that time, he attempted to have races with Gordon, but when the two met Connor and Caitlin during one of their runs, they called off the race. After the restoration was complete, Connor, Caitlin, Gordon and Spencer had another race from Ulfstead's grounds.
Spencer brought a VIP from the Mainland to present the Fat Controller with an award. Due to showing off, he ended up on Edward's Branch Line, annoying the Deputy Minister. However, he learned from his mistake and successfully brought the Deputy Minister back to Sodor the following day.
Spencer is very arrogant and pompous and considers himself to be above doing menial duties such as shunting. None of the engines like him, especially Thomas and Gordon. When he came to build the summer house, Spencer proved himself to be very nosey, sneaky, disobedient and rather neglectful. Despite being closely related, Gordon and Spencer have never gotten on very well with each other. This is mainly because of his pompous behavior, which exceeds even that of Gordon's. Although he has shown a kind side when he realises his mistakes. This side once came through when helped Percy find his way home to Tidmouth Sheds.
- Spencer met Pooh, Twilight and their friends in Pooh's Adventures of Thomas & Friends: Hero of the Rails and Tigger refers to him now as "Spencer-Boy" after he reformed.
- Spencer will meet Ash Ketchum, Littlefoot, SpongeBob, Simba, Tai and their friends in Ash's Adventures of Thomas & Friends: Hero of the Rails.
- Spencer is best friends with Fancypants.
- Spencer is the brother of Olwin, since they share the same basis.