He was probably connected to the family of Thomas Shannon (1801–73) of Yalobusha County, Mississippi. After moving with the family to Missouri, Shannon arrived in Washington County, Texas, to practice law. On April 7, 1856, in Yalobusha County, he married Arabella Abel. By 1857 he had moved to Anderson in Grimes County.
In Grimes County the twenty- three-year-old met with success. He continued his vocation of law and acquired a town plot for his practice, while his wife Bell gave music lessons. Two daughters, Nancy (b. 1858) and Anna (b. 1859) increased his family. At the same time, his business increased his social position. While he did own slaves, it appears that they were accepted in lieu of payment for his services. Two facts support this. He held no property outside of his town lot, and the number of slaves fluctuated notably. For example, Shannon owned five slaves in 1858, twenty-one in 1860, and seven in 1861.
When the Civil War erupted, Shannon wasted little time. He formed the Grimes County Rangers at Navasota in Grimes County on August 19, 1861, for a length of service of the entire war. Riding 200 miles to San Antonio, the unit arrived on August 31 where they were sworn in at the Plaza House and incorporated as Company C into the Fifth Texas Cavalry with Shannon placed as captain of the company. Soon the unit joined with Gen. Henry H. Sibley on an expedition to conquer New Mexico.
In the New Mexico invasion (see SIBLEY CAMPAIGN), Shannon was an aggressive leader in an army that earned the fearful nickname, "the terrible Texans." Indeed in one of the first engagements near Valverde, Confederate soldier William Davidson said this of Shannon: "Captain Shannon… was always ready to precipitate matters" in describing his calling for an attack two days before battle was to take place. At times a propensity for fearless action did get him into trouble. For example, on March 26, 1862, Shannon and his men were engaged in action near Glorieta Pass and found themselves "into kind of a pocket, so that the enemy were on three sides of him… but by hard fighting and good running, he finally got to us and our line…." Two days later his aggressive nature was the cause of his capture. Shannon became so blind in his pursuit of a retreating enemy, he "forgot that men on foot could not keep up with a man on horse back [sic] and by himself ran in among them," leading to his capture on March 28, 1862. His confinement was short however, as he was exchanged for a Captain Shivers of the Seventh United States. Infantry on September 17, 1862.
Major Shannon was reunited with his company just in time to begin preparations for actions against the Northern Armies in Louisiana. After a series of engagements in Western Louisiana, Major Shannon was promoted by right of seniority to lieutenant colonel on May 20, 1863. Again, his aggressive nature got the better of him as he was captured during the assault on Fort Butler near Donaldsonville on June 28, 1863.
A file in his service records erroneously states that he was killed during the battle, but his records indicate that he was taken into custody on June 29, 1863, and held in the customs house until transferred to the Steamer Clinton, which headed for New York City on September 20, 1863. After a quick processing at Fort Columbus in New York, he was sent to Johnson's Island, Ohio, on September 29, 1863. He remained incarcerated until February 19, 1865, when he was transferred to New Orleans in preparation for a prisoner exchange. Three months after his April 9 release, the war was over.
By the end of 1865, Shannon had returned to Grimes County and resumed his law practice in Navasota. However, times proved difficult. Although he had twenty acres and a horse in 1865, by 1866 he had sold his horse, and by 1867 he was reduced to only six acres. Records show his continued habitation in Navasota until 1871. After that he practiced law in Houston and Dallas. The date of his death is not known, but the best evidence is that he was killed by Indians near Wichita Falls in 1879, while on a surveying expedition.