Spencer is a sleek, privately owned engine belonging to to the Duke and Duchess of Boxford.
During his first visit to Sodor, Spencer took the Duke and Duchess of Boxford on a trip around Sodor, but 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 the quarry. When Spencer discovered Percy's mail trucks (thinking that Thomas was hiding them from Percy), he got into trouble with Sir Topham Hatt 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.
Spencer remained on Sodor for some time afterwards and helped Thomas with deliveries to the summer house, teased James for being pink, had a repaint at the Sodor Steamworks, and got covered in snow, thanks to Gordon.
Spencer later returned to the Mainland to carry out work for the Duke and Duchess of Boxford. However, Spencer came back to Sodor once again. During this trip, he delivered rubbish to Whiff's Waste Dump on Clean Sodor Day. He later showed Stafford around the Island.
Spencer is very arrogant and pompous and considers himself to be above doing menial duties such as shunting. When he came to build the summer house, Spencer proved himself to be very nosey, sneaky 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.
Spencer is an LNER Class A4 pacific. Mallard is also a member of this class.
Spencer was painted blue-grey with black lining. From the tenth season onwards, he was painted metallic silver.
- On the Steamies vs. Diesels DVD, Spencer is called a Diesel. This is because in the game, Spencer says he does not have a whistle, but an electric horn instead. However, his picture clearly shows him having a whistle.
- One of Spencer's models is currently on display in Canada at Nitrogen Studios.
- Spencer's TV series model was based on an A4 Pacific, however it was slightly modified. The smokebox was much less angled then it should have been due to the eye mechanism inside.