Diamantakos started his career at Atromitos Piraeus, from which he moved to the youth teams of Olympiacos in 2009.
In 2012, he moved to Superleague side Panionios on loan. In January 2013 he joined Aris FC on loan. He made 13 appearances without scoring any goal. In June 2013 he joined Ergotelis on loan. He did his best season so far by scoring 10 goals in 27 appearances in the league and in the cup.
Following his loan spell at Ergotelis, Olympiacos decided to keep him for the 2014–15 season. Diamantakos made his debut on 23 August 2014 with Olympiacos in the Superleague on a home game against Niki Volos and managed to score his first goal in the 28th minute with a close range strike. In February 2015, according to Russian reports, Arsenal Tula was close to signing Dimitris Diamantakos. With the Russian transfer window still open the striker reportedly was in the club's shortlist.
Overall, he had little playing time in his first season with Olympiacos, having to settle for a role mainly on the bench. He scored 4 goals in 17 appearances across all competitions helping his club on its route to winning the Greek Cup.
In August 2015, Diamantakos joined Karlsruhe for the season, again on loan. On 6 November 2015, in his fourth appearance he scored his first goal for the club scoring in a 3-0 home win against VfL Bochum. On 12 December 2015, he scored his third goal in the league, helping his club return to winning ways against SpVgg Greuther Fürth. On 18 December 2015, Diamantakos scored for the third time in a row, sealing Karlsruhe's win away to FC St. Pauli.
On 18 February 2016, Karlsruhe completed the permanent signing of the Greek striker for a reported transfer fee of €1 million. The transfer took immediate effect as the loan contract with Olympiacos was cancelled. On 20 February 2016, he scored the second goal helping his club to escape with a 2–2 home draw against Eintracht Braunschweig after teammate Ylli Sallahi had been sent off. On 3 April 2016, he scored in a 3-1 home win game against TSV 1860 Munich. On 2 May 2016, Diamantakos scored twice as Karlsruher SC beat neighbours SV Sandhausen 3–0 to record a third successive home win.
Diamantakos scored his first goal of the 2016–17 season on the second matchday, in a 1–1 draw against VfL Bochum. On 18 September 2016, he scored the equalising goal in a 1–1 draw against FC St. Pauli. On 19 November 2016, he scored the leading goal for Karlsruhe, as they dominated in the first half of the game, but lacked the force and determination in the attack,facing an unexpected 1-3 loss from SV Sandhausen. On 2 December 2016, was the fateful player as he scored to a penalty kick reducing the score, and missed the decisive penalty and the chance to tie up the game in a 2-1 home loss against Greuther Fürth. On 4 March 2017, he scored with a penalty kick sealing a 2-0 win home for his club against Hannover 96 in his club effort to avoid relegation.
On 10 June 2017, Bochum announced signing Diamantakos for the 2017–18 season.
As of 10 September 2017
He was a member of the Greek under-19 squad that earned second place in the 2012 UEFA European Under-19 Championship. Diamantakos made his international debut for Greece on 7 September 2014 against Romania. On 4 October 2016, Diamantakos is injured and will miss the two upcoming matches of Greece against Cyprus and Estonia.