He first led a company from Shawneetown Illinois that guarded the supply route from Vera Cruz to General Winfield Scott's Army.

He studied law, passed his bar exam, and used his legal license to help the claims of Mexican War veterans.

He led his men in the battles of Port Gibson, Champion's Hill, and Big Black River Bridge, and during the general assault of May 22, 1863 on Vicksburg MS, where troops under his command were the only Union forces to enter the Confederate works at the Railroad Redoubt where they planted the U.S.

For the rest of the war, General Lawler served as commander of the 1st Division, XIII Corps in Louisiana in the Department of the Gulf, taking command of the division during the disastrous Red River Campaign and leading it on an expedition in June 1864 to secure a crossing of the Atchafalaya River used by Confederate forces.

He died in the summer of 1882 and is buried in the Lawler Family Cemetery near Equality, Illinois, at the rear of the Old Slave House property.