Not much is known about Mickey's life prior to the story other than the fact that he has had some "run-ins with the law" in the past, mainly involving shoplifting among other things. It is unknown whether or not he had any ties to the criminal underworld. (The Crocodile's Dilemma)
Mickey is first seen along with his father and brother approaching Lester Nygaard on the sidewalk to pick on him. Mickey in particular refers to Lester as a "big stupid pumpkin." Mickey and his brother give their own side commentary to Sam and Lester's conversation, much to their annoyance.
He and his brother are later seen at Hess & Sons Transport playing with a box of inflatable sex dolls and begin fighting when Lorne Malvo walks in looking for Sam. Mickey takes charge due to him being the older brother which causes the two to continue fighting until Sam returns. After Malvo accuses Moe of being "a little dim" Mickey encourages his father to hit him. Mickey, along with the rest of Sam's entourage, stands in confusion as Malvo exits the building after taking a look at Sam.
After hearing the news of Sam's death, Mickey along with his brother and mother receive updates from Max Gold about Fargo's next move. Mickey receives a phone call from Malvo posing as Sam's attorney telling him all of Sam's inheritance went to Moe. Shortly afterwards, Mickey leads Moe outside where he attempts to beat his brother to death to secure his inheritance. Molly Solverson intervenes and tackles Mickey to the ground before he can finish the job. (The Crocodile's Dilemma)
Days later, Mickey and his brother are shooting crossbows at a "For Sale" sign as Lester pulls into their driveway. The two of them taunt Lester as he walks up to the front door until they are both dismissed by Gina. They return to shooting crossbows until Mickey sees his mother flirting with Lester. He loses concentration and shoots Moe in the backside with his crossbow. (A Muddy Road)
Mickey follows Moe and an angry Gina into Bo Munk Insurance Shop where the latter confronts Lester about being ineligible for Sam's life insurance policy. Mickey offers to beat him up for her but is silenced. Mickey is disgusted to learn of Gina's sexual history with Lester. Gina orders Moe and his brother to threaten Lester, only for Lester to fight back by putting staples in their faces. Gina eventually is talked down and storms out, the boys following behind her. (The Heap)
To be added.