His love of football and the financial difficulties of his family forced him to leave school early and split his time between the stadium and the barber shop in the village, where he worked as an assistant.

[9] His appearances for the "Queen of North" aroused the interest of the big teams of Thessaloniki, but also of the then coach of AEK Athens, Tryfon Tzanetis.

[10] The offers of the clubs of Thessaloniki did not meet the requirements of Veria and in fact his transfer to PAOK failed for a difference of 20,000 drachmas.

[8] In the summer of 1962, the 20-year-old Papaioannou was dressed in the yellow-black jersey, starting an integral course of 17 years that elevated him to one of the greatest players in the history of AEK Athens.

[8] The upcoming years for AEK and Papaioannou were almost ideal, as he showed that he was the one to take the leader role from Nestoridis in the team, pushing them as high as he could to lead them to success.

At the end of the season he won his first Cup with AEK, even though the final was scratched due to the suspension of the semi-finalists, Panathinaikos and Olympiacos from the competition.

Nevertheless, Papaioannou was willing to leave AEK under the given punishment of a one-year ban and then sign as free agent at Real, but the Spaniards refused, as they didn't want to disrupt their relations with the Greek club.

After two months and despite vocal competence of Papaioannou, Kazantzidis, realizing the "crime" of the departure of Mimis from football, persuaded him to return to AEK and mediated himself with the administration in order for the player to sign a very satisfactory contract of 500,000 drachmas per season at the start of December 1965.

[46] As AEK advanced in the second round of the European Cup, he scored with an impressive header, at their first away win in the institution by 2–0, for the 2nd leg against AB, securing the club's qualification to quarter-finals.

[58] From the previous season AEK was beginning to struggle financially and the administrative disputes appeared within the club, resulted in a series of issues.

Papaioannou continued to lead the club during the hard times, as they had a decent presence in the UEFA Cup, where they reached the second round and were eliminated by the great Liverpool of Bill Shankly.

In a competitive regeneration of the team under František Fadrhonc and with the transfers of Walter Wagner, Christos Ardizoglou, Tasos Konstantinou and Giorgos Dedes in their offense, Papaioannou raised his performances.

[65] In the summer of 1976 after the yellow-blacks were strengthened by the additions of Nikos Christidis, Takis Nikoloudis and the great Thomas Mavros, they played amazing football.

[66] In the episodic second leg on 29 September 1976 that was sent to the extra time, after AEK won a penalty in the last minute and Konstantinou who went to take it, given the order by Papaiannou to finish the job, calmly sent the ball into the net and the goalkeeper in the opposite direction.

[70] On 30 January, before the home match against Atromitos, Barlos awarded him with the golden double-headed eagle, which was the team's highest honor in those years.

[71] In the quarter-finals he helped his team tie the 3–0 defeat of the first leg against QPR, scoring a goal in the regular time and sending the match to penalties.

AEK Athens glorious campaign in the UEFA Cup ended after losing to the legendary Juventus of Giovanni Trapattoni, who eventually won the tournament.

[74] At the end of the season, with Papaioannou as their captain, they reached in the conquest of the domestic double with relative ease as they won the league with just 2 defeats and they went undefeated in the Cup, beating Panathinaikos in the second round, tearing apart Olympiacos by 6–1 in the semi-finals and prevailing against PAOK by 2–0 in the final.

[75] After a completely successful season, Barlos after taking the approval of Papaioannou[76] added to the star roster of the club the other "Mimis", by sigining the legendary captain of Panathinaikos, Domazos alongside Ferenc Puskás, as a coach, who both led the "greens" to the European Cup Final in 1971.

Even though he was approaching the age of 40, he attracted the interest of NASL clubs, but he had already decided to retire as a footballer at the end of the year, thus finishing his great career in 1982.

[7][88][93] After his return to Greece in 1986, he coached Kerkyra, Edessaikos, Olympiacos Chalkida, AE Kos and Pannafpliakos, respectively within 5 seasons.

[10][8] In January 1999, the combination of all the virtues and talents of Papaioannou were recognized by the IFFHS, where he was named the Greatest Greek Footballer of the 20th Century.

In front of approximately 2,000 fans of AEK, big names such as Gerd Müller (as a coach), Bernd Schuster, Dieter Burdenski, Holger Fach, Alois Reinhardt, Thomas Berthold, Bernd Förster, Ulrich Borowka, Rüdiger Abramczik, Michael Schulz, Miroslav Votava, Rudi Bommer, Walter Kelsch and Günther Herrmann were present.

[97] In December 2011, the autobiography of Papaiannou, entitled "Date in the Air" (Greek: "Ραντεβού στον Αέρα") was published by NIKAS publications and edited by Dimitras Apostolias.

[99] His name also is honoured on one of the four pillars of the Stadium, alongside other important figures of the club's history such as Kostas Nestoridis, Stelios Serafidis and Thomas Mavros.

[101] His funeral took place on 17 March at Nea Filadelfeia, where great figures of the past and the present of the Greek football attended to "farewell" him for the last time.

[103] On 19 March, in the home match against Panathinaikos, the matrix of the stadium showed clips from old interviews, while the team's anthem was playing and before the start of the game the fans of AEK raised a card stunt in his memory.

Οn 12 May 1965, at the 46th minute, during the 3–3 draw in a friendly match against Real Madrid, the central defender José Santamaría was stunned, not believing that the Greek player had just jumped "to the stars" and sent the ball into the net, making the score 3–1.

His playing intelligence and technical ability got him to be converted from a striker to an attacking midfielder, considering his physical condition at the time and in order to fit in with all the great players in the team's offense.

He adapted quickly in his new role, while the change of his position kept him in a high level, as despite his offensive skills, he also had good through passes and crosses, creating a lot of scoring chances for his teammates.