Until later in the series, it seems like Mallory has a different boyfriend (or at least love interest) in each episode. There are (for instance) episodes during the "Nick years" where Nick doesn't appear, but presumably they are still together.
Some have no specific timeframe. For example, Steven remembers Walter Murchison in the episode "Mr. Wrong".
|Not with My Sister You Don't||Eric Morrison|
|Big Brother is Watching||Ralph Rappaport|
|Death of a Grocer||Brad Hunter|
|A Christmas Story||Donny Hayes|
|Sherry Baby||Irwin "Skippy" Handelman (against her wishes)|
|Tender is the Knight||some other boy Steven is worried about (Jeff is away at Princeton, as we see in the next episode)|
|Ready or Not||Rick Harmon|
|Diary of a Young Girl||Kevin Jenkins|
|Love Thy Neighbor||Scott Handley (though Jennifer hates the idea)|
|Don't Kiss Me, I'm Only the Messenger||Mallory plants a huge kiss on Skippy (in an attempt to spare his feelings about Jane)|
|Higher Love||Mallory and Colin Spencer are attracted to one another, though Mallory is still in a relationship with Nick.|
|Matchmaker (filmed out of sequence)||Roger Sloate|
|Anniversary Waltz (filmed out of sequence)||
Kurt McMillan (a love interest)