[2][3][4] Halagian was born on 7 March 1939 in Bucharest, Romania, starting to play junior level football in 1950 at local club, Dinamo where he was formed and taught by coaches like Colea Vâlcov, Petre Steinbach and Ion Nedelescu.

[1][5][6] Halagian made 23 appearances in his first top-league season but the team relegated at the end of it, also during this time he was teammate with a young Nicolae Dobrin.

[5][7] His first game managed took place on 21 August 1971, ending with a 2–1 home win over Rapid București, courtesy of a brace scored by Radu Jercan.

[1][2][5][6] Afterwards, the club participated in the 1972–73 European Cup, eliminating Aris Bonnevoie in the first round, then in the following one they won a home game with 2–1 against Real Madrid led by coach Miguel Muñoz but lost the second leg with 3–1.

[11][14] In the same season he helped the club win the second title in its history which was mathematically won after a spectacular 4–3 away victory in the last round over Dinamo București.

[1][2][5][6][8][13][15] Next season, Argeș participated in the 1979–80 European Cup where after passing AEK Athens in the first round, they got eliminated by Brian Clough's Nottingham Forest who were the title holders and eventual winners of the competition.

[13][18] In 1984 he arrived at Steaua București where he worked with some of his players from Olt Scornicești like Ilie Bărbulescu, Marin Radu, Victor Pițurcă and Adrian Bumbescu but after the first seven rounds of the season in which the team was undefeated and an elimination suffered in the first round of the European Cup Winners' Cup with 1–0 on aggregate in front of A.S. Roma led by coach Sven-Göran Eriksson, Halagian was replaced with Emerich Jenei, the team managing to win the title at the end of the season.

[2][10][21][22] In the spring of 1985, Halagian takes over Universitatea Craiova with whom he finishes the season on the fourth place and reaches the 1985 Cupa României final which was lost with 2–1 in front of his former team, Steaua.

[1][25] Halagian left Pitești after three years to coach Victoria București with whom he finished the 1988–89 season on the third place, also eliminating Sliema Wanderers, Dinamo Minsk and Turun Palloseura in the 1988–89 UEFA Cup edition, the campaign ending in the quarter-finals where they were defeated by Dynamo Dresden.

[1][13][21][30] Halagian also led The Bankers in the 1996–97 UEFA Cup edition, eliminating Partizan Belgrade and Chornomorets Odesa in the first rounds, the campaign ending after a 3–1 aggregate loss in front of Club Brugge.

[33] In April 2011, Halagian had the last coaching spell of his career, returning at Gloria Bistrița, but was sacked three weeks later because of issues regarding players shaving before matches.

[10][21][36] It was said about him that he was a strict and severe manager, having expectations for the footballers such as shaving before the matches but his reply in an interview was:"I don't think I was a rough coach because I loved all my players, no matter how good they were.

"[7][10][37] He also had superstitions, many years wearing a Chicago Bulls hat during the games because he believed it would bring him luck:"Even now I have a bunch of caps in my hanger.