Sam McGuire is Lizzie and Matt's father. He could be best described as a bit goofy and quite clueless about raising his kids. However, he always tries his best to help Lizzie out and understand her, and is always available to lend a helping hand.
Sam is often clueless about what his children are really up to, leaving it to their mother (Jo McGuire) to deal with issues of children and family. But he sometimes enjoys clever schemes of the children when he finds out about them (to the distress of Jo, who may find them to be a bad idea). He is portrayed with a good deal of clumsiness - something which his daughter Lizzie apparently inherited.
Sam pretends to hate the fact that Lizzie is growing up and tries his best to stop her. However, in the Season 2 episode Bye Bye Hillridge Junior High, he shows he has learned to accept that growing up and getting older is inevitable when he tells Lizzie that she grew up in middle school. He also tells her about all those things he remembered her doing (running for class president, rhythmic gymnastics, helping to save the environment and getting her first job). After she explained to him that she failed at every one of those things, he assures her that she tried her best, and that the only time she failed at something was when she didn't try. He then tells her he thinks she will do just fine in high school.
In "Gordo's Bar Mitzvah", he said he was 15 1/2 years old in 1976, which would put his birth date around 1961. He is stated to have been a nerdy child and teenager, president of the Audiovisual Club, a Mathlete, and a hall monitor. He built a soap box racer which he still had in a shed outside the house as an adult, and his aptitude test revealed him to have "untapped mechanical ability". He is afraid of heights because of a "bad rope-climbing incident", as revealed in the episode "Party Over Here".
In the Season 1 episode "Obsession", when Lizzie sarcastically asked how many trash cans Matt would get stuck in, Matt replied, "See? Even my lame-o sister knows what's coming!"
Jo told Matt that he "should make the best of" being a hall monitor. Sam agreed, telling Matt that being a hall monitor was a position of honor. But then, after he remembered when he was a hall monitor (as well as also getting put in a trash can), he then said to Matt, "Your sister may be right, son".