He joined the United States Navy Reserve as a Direct Commission Officer in the Intelligence career field in 1989 and was recalled to active duty for the 1999 NATO bombing of Yugoslavia.

[12][13] While Kirk was an undergraduate student at Cornell University he held a work–study job supervising a play group at the Forest Home Chapel nursery school.

After leaving Capitol Hill in 1990, he worked at the World Bank and as an aide at the State Department on the Central American peace process.

VAQ-209 was combined with three other EA-6B squadrons to form an ad hoc unit called Electronic Attack Wing Aviano, Italy.

[20] In a speech on the floor of the House in 2003, Kirk stated: "The last time I was in Iraq I was in uniform, flying at 20,000 feet, and the Iraqi Air Defense network was shooting at us".

[21][22] During his tenure in the military, Kirk was twice counseled by the Pentagon, after incidents in which he was accused of conducting political business while on duty.

On one occasion Kirk commented on Rod Blagojevich's arrest and posted a tweet while on duty with the Navy in Afghanistan.

[23] According to the Pentagon, Kirk was required to sign a statement acknowledging he knew the rules before returning to active duty.

[25] In 2010, Kirk corrected statements he had made about being awarded "Navy Intelligence Officer of the Year" after it was brought to the media's attention by his Democratic opponent, Alexi Giannoulias.

[26] In a 2002 House committee hearing recorded by C-SPAN, Kirk said, "I was the Navy's Intelligence Officer of the Year", an achievement he said gave him special qualifications to discuss national security spending.

[27] In May 2010, The Washington Post reported that Kirk's claim to having been named the Navy's "Intelligence Officer of the Year" was erroneous.

[18] Kirk later apologized for this and other errors, including a claim made by his office of having participated in Operation Desert Storm when in fact he did not.

[29] On June 7, 2010, Medal of Honor recipient and advocate of Veteran's benefits, Allen Lynch, deemed Mark Kirk's apologies adequate, and further commented: "To me, in my opinion, it's just a bunch of nit picking.

"[36] In 2006, Kirk pushed for an expansion of O'Hare and worked with Rahm Emanuel on a package to clean up Lake Michigan.

"[41] In 2012, Kirk's ex-wife accused him of concealing a payment of $143,000 to a former girlfriend, Dodie McCracken, who had worked on his 2010 U.S. Senate campaign.

[50] Kirk's campaign purchased television air time to advertise his opposition to admitting refugees to the United States "until it can be done safely".

[1][2] On June 7, 2016, Kirk withdrew his initial support for businessman and presumptive Republican nominee Donald Trump in the 2016 presidential election due to his "past attacks on Hispanics, women and the disabled like me."

[62] Kirk and Durbin also worked together to bring $186 million in federal funds to support improved rail service from Chicago to St. Louis.

[63] Kirk authored legislation, entitled the Lincoln Legacy Infrastructure Development Act, that sought to eliminate barriers and encourage private investment in roads, transit, airport and rail.

[69] In 2016, Kirk suggested that Iran should be required to provide reports about how funds made available through sanctions relief were used to ensure that money was not ending up in the hands of Hezbollah or the Iranian military.

[73] Kirk and Representative Bob Dold (R–IL-10) (who succeeded him in his House seat) introduced bipartisan legislation to expand qualifications for ending federal pension payouts to elected officials convicted of corruption.

[78] He has a lifetime 75% grade from Planned Parenthood and an 80% rating in 2015 from NARAL Pro-Choice America, both organizations that support legal abortion access.

The bill would amend the Fair Labor Standards Act of 1938 (FLSA) to increase the federal minimum wage for employees to $10.10 per hour over the course of a two-year period.

[91] In March 2015, Kirk voted for an amendment to establish a deficit-neutral reserve fund to allow employees to earn paid sick time.

[1] He supports background checks for gun sales,[94] and in 2013 was the only Republican senator to vote for an assault weapons ban.

Senator to publicly state that President Barack Obama's eventual replacement nominee for the Supreme Court should get a hearing and a vote.

[100] In April 2016, Kirk met with Obama's nominee, Merrick Garland, and circulated a memo to his Republican colleagues encouraging them to meet with him as well.

[106] Two days later, he underwent neurosurgery at Northwestern Memorial Hospital in Chicago; a piece of his cranium was temporarily removed to lessen any danger from the brain swelling process.

[106] He suffered significant left-sided weakness and spent several months at an inpatient physical rehabilitation center.

On November 4, he participated in a "SkyRise Chicago" challenge to climb the stairs of Willis Tower, successfully completing 37 floors.