Jackson sent Major William Lawrence and a contingent of 160 regular soldiers to repair and defend the dilapidated fort.

From The Battle of New Orleans: Andrew Jackson and America's First Military Victory by Robert V. Remini.  Page 19.

On arriving at Mobile, Jackson sent Major William Lawrence and a contingent of 160 regular soldiers to repair and defend the dilapidated fort. Working at top speed, they managed within two weeks to bring the fort to an acceptable level of defense. Then on September 12 a British force of 225 marines and Indians was put ashore nine miles east of the fort and a naval squadron consisting of the Hermes, Carron, Sophie, and Childers with a total of seventy-eight guns under Captain William H. Percy arrived at Mobile Bay to begin the land-sea invasion. The Hermes and Sophie got within range of the fort and opened up with their heavy guns at 4:20 P.M. But the shallowness of the channel and the dying wind made it impossible for these ships to maneuver. By 7:00 P.M. the Hermes had gone aground with her sails shredded and her rigging shot away by the returning fire from the American fort. After transferring his men to other ships, Percy abandoned the Hermes and set it ablaze. The resulting explosion of her magazine could be heard by Jackson thirty miles away in Mobile. The destruction of the Hermes marked the end of the engagement.

Realizing that he could not capture the fort, Percy withdrew and sailed back to Pensacola. The marines and Indians who had landed got within a thousand yards of the fort and attempted a feeble assault, but when they saw the fleet sailing off they turned around and retreated to Pensacola. In the engagement Nicholls was wounded in the leg and blinded in one eye. The British lost twenty-two dead aboard the Hermes and twenty wounded; the Sophie had nine killed and thirteen wounded, while the Carron sustained one killed and four wounded. Lawrence lost only four killed and five wounded. 

The loss of this battle was catastrophic for British plans—and it could have been prevented. The strategy of a land-sea operation to capture Mobile and thereby provide the location for a massive invasion by the army gathering in Jamaica had great merit and should have been better planned and organized. Defeat resulted because shallow-bottomed boats were needed to properly navigate the waters of Mobile Bay so as to get into position to bombard the fort. In addition, the land force was too small for the task of capturing the and faced a highly disciplined and well-entrenched contingent of American soldiers.