He married Nancy Watts in 1827 and kept a store on the square in Nacogdoches until 1830. He made one trip to New Orleans and returned by keelboat up the Neches and Angelina rivers, but mainly he took cotton and piece goods to Saltillo and traded them for livestock and specie. In 1830 he moved to San Felipe and continued trading to the south, sometimes in partnership with the sons of Jared E. Groce. He also maintained an interest on the lower Trinity, where Michael B. Menard developed a sawmill. In 1834 he became senior partner with Samuel May Williams in McKinney and Williams, a firm located on the Brazos; Williams supplied the bookkeeping and commercial contacts in the United States, while McKinney collected and shipped the cotton. The firm developed Quintana at the mouth of the river in 1835 and used its credit to help finance the Texas Revolution to the amount of $99,000, which was never repaid in full.
Always impetuous and ready for a fight, McKinney, on board his schooner, San Felipe, captured the Correo de Mexico in September 1835. The Mexican vessel had been preying on Texas-bound shipping. McKinney obtained a privateering licence from the Provisional Government and used the firm's credit to buy the William Robbins, renamed Liberty, for the rebel government. Though he refused commissions as commissary general and loan agent, he continued to forward men and supplies to the Texas army.
He and Williams joined Menard in 1833 in a scheme to claim Galveston Island, and in 1836 they combined with others to secure a charter for the Galveston City Company. The firm had a wharf and warehouse on the island in October 1837, when Racer's Hurricane struck and severely damaged their property. McKinney built a house for Williams and an identical one for himself west of town in 1839, but he lived in his home only briefly before his marriage ended. In 1843 he secured a divorce and the same year married Anna Gibbs, a native of Boston. There were no children from either union.
McKinney withdrew from the partnership with Williams in 1842 and devoted himself to trading and stock raising, first on the island, where he had a race course, and in 1850 in Travis County, where he constructed a fine stone house, a gristmill, and another quarter horse track opposite the capital city. He also served as state senator from Galveston in 1846 and as representative in 1849. He was a member of the Democratic party and a Unionist in 1860-61. He had opposed independence, annexation, and secession, but once each was accomplished, he worked to support the government. He served the Confederacy as a special cotton agent and made several trips to Mexico with cotton, but the duplicity of various individuals and the confusion of the times left him liable for contracted debts. This burden, along with the loss of about fourteen slaves, crippled him financially. His once-large estate was reduced to $5,000. He died on October 2, 1873, after a long struggle with a kidney disease, and was survived by his wife. His ranch became McKinney Falls State Park in 1976.
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