"7 Little Johnstons" star Anna Johnston was hauled into jail after being arrested for DUI ... and flashed a big smile for her mug shot.

According to a police report obtained by multiple outlets, Anna was arrested in Georgia on Saturday after cops saw her car parked on the side of the highway and investigated.

The report says Anna was driving her boyfriend, Jeffrey Messervy, and his cousin when she got into an argument with Jeffrey and pulled over. They say Jeffrey told them he was "upset due to Anna driving them around with alcohol in her system."

Anna was allegedly slurring her speech when the officer arrived and was asked to take a breathalyzer ... and cops say she blew a 0.139. Police say she reeked of booze and bombed a field sobriety test, so they arrested her for DUI.

The day after her DUI bust, Anna was released from the local jail on $1,563 bond.

The Sun reports Jeffrey had a warrant out and was also arrested … and he's apparently being held on a $47,350 bond for 13 counts of misdemeanor forgery.

Anna left "7 Little Johnstons" after season 16 because of her rocky relationship with her family.