At the age of 15, Shlemenko seriously started to train ARB, which is the Russian abbreviation for "army hand-to-hand combat".

During the early stage of his professional MMA-career, Shlemenko competed in the IAFC (International Absolute Fighting Council).

In May 2005, after only one year of his professional MMA career, Shlemenko had an impressive 15–2 record, and was gaining more and more attention by the Russian fans and the media.

[citation needed] In April 2006, Alexander faced the future Strikeforce Middleweight Champion Ronaldo Souza at Jungle Fight 5.

As Alexander stated during an interview in 2010, the hardest fight of his career was the rematch with Jose "Pelé" Landi Jons in September 2006.

The only knockout loss of his career Alexander received from Jordan Radev, a Bulgarian Olympic wrestler and mixed martial artist.

Lombard controlled nearly the whole fight, landing elusive striking combinations and heavy elbows from Shlemenko's guard.

Despite taking a lot of damage that would've finished most opponents, Shlemenko actively fought back and even won the 5th round on the judges scorecards with a few well placed knees, some spinning backfists and stuffing takedown attempts.

This was due to Hector Lombard relying mostly on takedowns to win the rounds after Shlemenko figured out his timing on the feet.

In the opening round of the Bellator Season Five Middleweight Tournament, Shlemenko fought Zelg Galesic, a Croatian mixed martial artist with a Tae Kwon Do background.

Despite these expectations, Alexander won via standing guillotine choke in the first round, proving his rarely seen submission skills.

A landed backfist, followed by heavy knee strikes to the head brought Rogers in trouble in the first half of the round.

After both fighters exchanged some punches, Alexander managed to land another unanswered series of heavy knees to the head of Rogers, which forced the referee to stop the fight at 2:31 of the second round.

However, on 19 August, Marshall was forced out of the title bout due to an injury and was replaced by Season 8 Middleweight Tournament runner up Brett Cooper.

Shlemenko won the fight via knockout in the first round with a body punch, successfully defending his title for the second time.

After his loss to Tito Ortiz, Shlemenko was expected to face Yasubey Enomoto at Fight Nights: Battle of Moscow 16 on 11 July 2014.

Shlemenko faced fellow knockout artist Melvin Manhoef in the main event at Bellator 133 on 13 February 2015.

On 17 March 2015, Shlemenko was suspended indefinitely for failing a post fight drug test with elevated testosterone levels.

[18][19] In June 2015, the California State Athletic Commission (CSAC) suspended Shlemenko for an unprecedented three years with a $10,000 fine.

[25] After the stint in Bellator, Shlemenko faced Jonas Billstein at his native Russian Cagefighting Championship 5 on 15 December 2018.

Shlemenko faced Chris Honeycutt at Eastern Economic Forum: Roscongress Vladivostok Combat Night on 15 September 2019.

[34] Shlemenko rematched Aleksandar Ilić on 15 December 2023 at RCC 17, avenging his previous loss by TKO stoppage at the end of the first round.

[38] Shlemenko is known for his unorthodox striking style, which is difficult to refer to as traditional Muay Thai, Kickboxing, or ARB.

Often esteemed as a pure striker, Shlemenko also has some grappling skills, which he showed by defeating via submission among others Gregory Babene and Zakir Lalashov[39][40] (both via triangle choke).