Jeffress was booked for disorderly conduct, assault and criminal damage on Jan. 4. The latter two charges were dismissed on Jan. 24 in Surprise Municipal Court.
He was sentenced to serve 20 hours of community services for the disorderly conduct charge and to attend a domestic violence counseling class.
The arrest was made public Friday and Jeffress issued a statement through the Royals, saying "There was an argument between myself and my girlfriend and I lost my temper, but I never put my hands on her."
Jeffress said a call was placed to the authorities due to property damage.
In the statement, he apologized to the people of Kansas City, the Glass family, which owns the Royals, general manager Dayton Moore, manager Ned Yost and his "peers."
"There are consequences in what you do or say. I've disappointed my family and the name on the back of my jersey. I just want to make it clear this is not me."
Jeffress was acquired from the Milwaukee Brewers in the Zack Greinke trade before the 2011 season. He began last season in the Kansas City bullpen, but was sent to the minors May 19 with a 1-1 record and 4.70 earned run average in 14 games.
"We've spoken to all parties, including local law enforcement officials, and we stand behind Jeremy at this point based on the information we have received," Moore said. "Jeremy has a good heart, and we understand and appreciate he's trying to manage his personal life in a more honorable way."
While in the Brewers organization, Jeffress was twice suspended for violating the minor league drug policy. He admitted those suspensions were for marijuana use. The first suspension was 50 games and the second 100.
He was the Brewers' 2006 first-round draft pick, the 16th overall selection, out of Halifax County High School in Virginia.