Spencer is a sleek streamlined engine privately owned by The Duke and Duchess of Boxford. He is Mallard's brother.
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; however, he blamed the incident on 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 line 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 a 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 Sodor Slate Quarry. When Spencer discovered Percy's mail vans (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 turned up for their holiday. He then got stuck in the mud when he chased Thomas to Knapford. After being rescued by Hiro, Spencer apologised to Thomas and Hiro and they all worked together 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. Once he finally arrived at Knapford, the Deputy Minister crossly announced that he would not travel back to the Mainland with Spencer. Throughout the day, as the other engines were showing off, Spencer felt sorry for himself and wished he had done his work properly. He learned from his mistake and successfully brought the Deputy Minister back to Sodor the following day.
One day, Spencer was travelling through Henry's Tunnel and almost ran into Thomas, who was hiding in the tunnel pretending to be a made-up engine called Geoffrey, to trick the Fat Controller. At winter, he had to stay on Sodor because the Vicarstown Bridge was blocked with snow. He teased Henry by telling him he might see an abominable snowman. But when the two engines saw something covered in snow walking towards them, Spencer tried to run away, but his valves became blocked. Luckily, it was only the Fat Controller, who had gotten into trouble in the snow. Later, Spencer had to take the Duke and Duchess to Callan Castle, when his valve gear had snapped and broke down, so Thomas, Bertie and Harold had to take the Duke and Duchess to Callan Castle instead.
Spencer would then go on to participate in The Great Railway Show where he served as a UK representative and competed in The Great Race event against his cousins, Gordon and Flying Scotsman, as well as Axel and Etienne. He came in second place, narrowly losing to Etienne.
In the twenty-first season, he and Gordon love to have a race who is the fastest. However, he on the platform one that Gordon was on platform one before, instead he going on platform two. Then, Thomas and Philip shunted their coaches to him and Gordon for wrong platform mixed up and sets off without realize that they had a wrong coaches. After they raced along the main line, Gordon stops at Wellsworth with Spencer's Coaches while he thundered down with the express that the upsetting passengers is missing their stops until they stop at Callan Castle and they didn't realise that they had a wrong coaches that they shunted on the platform for the first place. Then, he and Gordon takes the passengers to the right stations and returned to Knapford and apologised to each other as they watched the fireworks all together.
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 behaviour, which exceeds even that of Gordon's. Despite this, he has shown a kind side when he realised his mistakes. This side once came through when he helped Percy find his way home to Tidmouth Sheds. He also wants to be really useful too like all the other engines.
Spencer is based on an LNER Class A4 Pacific. Mallard, holder of the world speed record for steam locomotives, is another member of this class. An additional five members of this class survive in preservation.
Spencer was painted blue-grey with black lining. From the tenth season onwards, he is painted metallic silver, still with black lining. He has red nameplates with his name in gold on the sides of his boiler. His livery may have been inspired by that of the first four A4s, which were painted in a "silver" livery.