McCulloch joined the Texas army under Sam Houston in its retreat to east Texas. Assigned to Captain Isaac N. Moreland's artillery company at the Battle of San Jacinto (April 21, 1836), he commanded one of the "Twin Sisters"—two six-pounder cannon sent to aid the Texans by the citizens of Cincinnati. One of the twin sisters was named Eleanor, the other Elizabeth. It is believed he chose to command Elizabeth to honor his dear friend and mentor, David Crockett, whose widow was Elizabeth Crockett. He made deadly use of his cannon against the Mexican positions and received a battlefield commission as first lieutenant. For his service (dating before April 18, 1836), McCulloch was issued Texas Bounty Certificate No. 2473 for 320 acres (1.3 km²). In 1839, he also received Donation Certificate No. 776 for 640 acres (2.6 km²), for his service at San Jacinto.