At his young age, Lawrence fought in the disastrous Battle of Mohi on 11 April 1241, where King Béla's royal army were severely defeated by the invading Mongols. According to historian Jenő Szűcs, he belonged to Béla IV's accompaniment, who fled Hungary through Transdanubia, escaping from the Mongols. The young Lawrence entered court service there and remained a member of the escort in Dalmatia, where Béla and his family took refugee in the well-fortified towns on the coast of the Adriatic Sea. Following the withdrawal of the Mongol hordes in the next year, Lawrence was commissioned to restore order in Western Transdanubia and defend the borderlands against the incursions of Duke Frederick the Quarrelsome, who previously forced Béla to cede three counties (most probably Locsmánd, Pozsony, and Sopron), taking advantage of the Hungarian monarch's desperate situation, who fled from the chasing Mongol detachments. It is plausible that Lawrence participated in the royal campaign in the second half of 1242, when Béla IV invaded Austria and forced Duke Frederick II to surrender the three counties ceded to him during the Mongol invasion. It is also possible that Lawrence was actively involved in the various military expeditions to Austria in the 1240s (for instance, Battle of the Leitha River) and 1250s, which also led to the Hungarian occupation of the Duchy of Styria in 1254. Lawrence Aba was created "ispán" of Sopron County in 1257. He served in this capacity until 1270, the death of Béla IV.