So I am US citizen and I want to travel to the UK but I heard you have to wait 12 months after you have gotten off probation. My misdemeanor only came with a fine, but I wanted to get it removed from my record, so I asked for a withheld judgment which they gave me 6 months of unsupervised probation before I could apply to have it dismissed from my record. So my trip date falls 3 months short of a year from the end of that probation. I want to know that since I got the misdemeanor dismissed will that affect me? Should I wait the year or can I go earlier?
I heard you have to wait 12 months after you have gotten off probation
There are no "have to"'s whatsoever in this regard. You don't get a ban due to probation.
At the end of the day, a single misdemeanour is not normally a cause for concern. To be safe, you could bring documentation about your misdemeanour and probation, and if, by chance, the UK border officers ask about convictions, tell them the truth.
Bottom line: yes, you can visit the UK immediately if you want.