Before entering politics, Shirey built a distinguished career in healthcare spanning over 30 years. He began his professional journey as a microbiologist, working in that field for approximately 10 years in Tuscaloosa before transitioning to optometry. Finding microbiology fascinating but isolating, Shirey yearned for patient contact and took a part-time job with an optometrist in Mobile to explore the field. This experience led him to pursue a career change, enrolling at Southern College of Optometry.

Shirey earned a Bachelor of Science degree in Medical Technology from the University of Alabama at Birmingham and subsequently obtained his Doctorate of Optometry from Southern College of Optometry in Memphis, Tennessee. During his optometry education, he received The Outstanding Clinician Award in his second and fourth years and the Bausch & Lomb Surgical Award. He was also inducted into Beta Sigma Kappa, an academic honorary society.

Since 2005, Shirey has been the CEO and owner of Southern Vision Care in Mobile, Alabama, a medically oriented practice covering a wide range of ocular disease as well as general optometry. The practice, which he operates with his wife Mary as practice administrator, has been serving the Mobile community for nearly 20 years. Shirey is certified in Diagnostic Pharmaceutical Agents as well as Therapeutic Pharmaceutical Agents and is currently the only board certified optometrist in Mobile, Alabama, having been certified by the American Board of Optometry in 2011.

Throughout his optometry career, Shirey has been actively involved in professional organizations and advocacy work. He served as president of the Alabama Optometric Association and was named "Young Optometrist of the Year" by the organization in 2008. During his presidency, the association successfully passed a "non-covered services bill" and tax regulations dealing with how optometry products are taxed in Alabama. He has also presented lectures ranging from billing and coding to advanced imaging and spent time on Alcon's speaker's bureau.

Shirey's political involvement began years before his election to the state legislature. For approximately 20 years, he was actively engaged in Montgomery legislative sessions through his work with the Alabama Optometric Association, serving as an advocate for the profession. He also worked on local campaigns, helping with activities such as knocking on doors and putting up yard signs. In 2018, he made his first bid for elected office, running for the Republican nomination for Alabama Senate District 34 but losing to Jack Williams with 36.1% of the vote.

Following his unsuccessful Senate race, Shirey announced his candidacy for the Alabama House of Representatives District 100 seat when the incumbent decided not to seek re-election. In the 2022 general election, he defeated Libertarian opponent Peyton Warren with 85.8% of the vote, receiving 9,741 votes to Warren's 1,609. His current term ends on November 4, 2026.

In the Alabama House of Representatives, Shirey serves on several committees including Health, Commerce and Small Business, Constitution, Campaigns and Elections, and Mobile County Legislation. His committee assignments reflect his healthcare background and business experience. As a legislator, Shirey has sponsored multiple pieces of legislation, including bills related to taxation, healthcare, and local governance issues.

One of Shirey's most significant legislative initiatives has been his advocacy for removing the state sales tax on optical aids, including eyeglasses and contact lenses. He introduced House Bill 176 in 2025, which would exempt these items from Alabama state sales tax, noting that Alabama is one of only five states that tax these essential vision aids. The bill passed the Alabama House unanimously in April 2025. Shirey has also sponsored legislation related to sheriff authorization for credit card use (HB101) and Mobile County election worker compensation (HB180).

In his personal life, Shirey is married to Mary Shirey, who has been involved in the optometric field since 1996 when they moved to Memphis for his doctorate studies. Mary serves as practice administrator for Southern Vision Care and previously worked as an ophthalmic technician. The couple has one daughter, Cate, who graduated from the University of Alabama with a Communications degree. In December 2022, Mark and Mary celebrated their 27th wedding anniversary.

Outside of his professional and political duties, Shirey enjoys various recreational activities including golf, hunting, fishing, and aviation. He is a certified scuba diver and has several hours of flight training, having completed his first solo flight on October 26, 2008. He has expressed intentions to eventually complete his private pilot's license when time permits. Shirey describes himself as a fiscal conservative who supports smaller government and believes in making tax cuts while reducing government size accordingly.