Her first appearance was in season 2 episode "The Bat Mitzvah".
At the beginning of Season 4, Eric said that he and Sloan are "taking a break." In the middle of Season 4, it is shown that they actually broke up in the Season 3 episode.
At the end of Season 6, Eric and Sloan get back together and become engaged.
However, they have a falling out when Eric teams with his friend and co-worker Scott Lavin to take over their managing agency that Sloan's godfather, Murray Berenson, owns and founded although they eventually reunite, presumably for good in the Season 8 finale, The End.