In 1986, Brian was summoned to Tampa by Sharon, who had discovered that Susan was posing as her, as part of Nicky and Mary Grand's scheme to get Sharon hooked up with Mary's father, Bill. Sharon wanted his help to give Susan a taste of her own medicine.
Together with Sharon in disguise, Brian goes to a restaurant, where Susan was having a date with Bill as "Sharon". Him being there with a mysterious woman convinced Susan that he was cheating on her, causing her to break character and confront him.
Brian later attended Sharon and Bill's wedding, together with Susan. They divorced some time prior to her meeting and marrying Jeffrey Wyatt.
In The Parent Trap II (1986), Susan mentions that she's been married to Brian for 12 years. This would place the wedding in 1974. However, in The Parent Trap III (1989), Susan gives her former step-children's (Brian's kids) current ages as 14 and 16. Which would place the birth of the two children in 1975 and 1973.