During the 1997–98 season, while playing in the youth championship, goalkeeper Cristian Munteanu was sent-off, and since there were no substitutes left, Niculae replaced him without receiving any goal from Petrolul Ploiești.
[5] On 22 November 1996, Niculae became the third youngest-ever player to debut in Divizia A, at 15 years and 190 days,[6] after coming on as a substitute in Dinamo's 5–2 win at home against Farul Constanța.
[39] On 3 May, Niculae scored a brace against FC Argeș[40] and he also wore the captain's armband for the first time for the closing stages of the match, after both Ioan Lupescu and Adrian Mihalcea were substituted off.
[5] Niculae won his first major trophy of his career as Dinamo bagged their first top-flight title in eight years, by a twelve-point margin,[41] contributing with 13 goals in 27 appearances.
[8] Dinamo manage The Double on 13 May, as they won 2–0 against Universitatea Craiova in the Romanian Cup final, with Niculae opening the scoreboard from the penalty spot in the 31st minute.
[59] He helped his team to reach the Romanian Cup final, netting in both legs of the semi-finals against Sportul Studențesc, Dinamo claiming a 5–3 win on aggregate.
Niculae dedicated his goals to former team captain Cătălin Hîldan, who died on the pitch in a friendly match against Șantierul Naval Oltenița on 5 October 2000.
[75] He returned on the field following his one-match ban on 8 September,[76] winning a penalty that gave Sporting a 1–0 lead against Leiria, after Mário Jardel converted from the spot.
[86] On 22 December, in a game against Vitória de Setúbal, after a dangerous challenge, he suffered a career-threatening knee injury, ending his season prematurely.
[87][88] Despite László Bölöni intended to give Niculae his come-back on the field faster than anticipated on Sporting's last game of the season against Beira-Mar, the doctors eventually denied the player to return.
[90] After a long recovery, Niculae finally returned to regular first team action in the match against Leixões, scoring Sporting's second in a 5–1 win in the Portuguese supercup.
[95] He returned on the field on 2 February 2003, in a 4–0 home win against Paços Ferreira, coming off the bench in the 73rd minute to assist Mario Jardel's third goal and Sporting's fourth.
[101] He scored again on 10 April, in a 4–0 home victory against Estrela Amadora, coming off the bench in the 75th minute[102] On 9 May he opened the score-sheet in the last league match of the season against Vitória Guimarães, in a 2–0 away win.
[105] He came back on the field after a five-month break on 19 February 2005, in a goalless draw with Leiria, coming as a substitute for Ricardo Sá Pinto in the 81st minute.
[107] On 14 April, he scored what it proved to be his final goal for Sporting, equalizing with a powerful header in a 4–1 home win against Newcastle United from the UEFA Cup quarter-finals.
[121] After spending six months as a free agent, Niculae signed a six-month deal with German Bundesliga side Mainz 05 on 12 December 2006 where he was wanted by coach Jürgen Klopp.
[122] After he passed his medical, Niculae joined Scottish Premier League's Inverness Caledonian Thistle, signing a two-year contract on 18 July 2007.
[133] He continued his excellent form in January 2008, netting another brace and obtaining a penalty in a 3–0 victory against Gretna, giving Inverness its fifth home win in a row.
[136] After the resignation of chairman Alan Savage, it became clear Inverness could not afford Niculae's wages,[137][138] and the club accepted an offer of €500,000 from Dinamo Bucharest, in the summer of 2008.
For the last ten minutes of the match, since there were no substitutes left, Niculae replaced the injured goalkeeper Bogdan Lobonț, managing to keep a clean sheet.
[162] On 3 December, Niculae scored the two goals that brought Dinamo the victory against Sturm Graz in a 2–1 home win from the Europa League group stage.
[187] He continued his excellent form in the third round, scoring once from the penalty spot and providing one assist for Ionel Dănciulescu in a 2–0 win against Pandurii Târgu Jiu.
[194] Twelve days later he converted a controversial penalty kick,[195] in a 1–0 victory against FCM Târgu Mureș, giving Dinamo an end of 2011 as winter champions.
[220] He made his Chinese Super League debut by playing 81 minutes in the 1–0 victory against Dalian Aerbin from the first round of the season, scoring his first goal one week later in a 2–0 win over Changchun Yatai.
[222] In September 2013, Marius Niculae signed with Hoverla Uzhhorod where he was teammates with fellow Romanians Răzvan Cociș, Cristian Oroș and Alexandru Dandea, making his Ukrainian Premier League debut on 5 October when he appeared for the final minutes of the 0–0 with Zorya Luhansk.
[226] However, only one month later, without playing a single game, he and the club ended their collaboration, Niculae claiming it was a problem of financial order as he did not receive all the money promised from them.
[241] On 6 October, Niculae played his final match for the U21 squad, netting Romania's only goal in the 1–1 draw against European Championship holders Italy.
[87] On 28 April 2004, Niculae returned at the national team, coming as a late substitute in a 5–1 thrashing win against Germany, the worst away defeat for the Germans since the past 65 years.
[260] His last appearance for the national team took place on 19 November 2013 in a 1–1 with Greece at the 2014 World Cup play-offs, failing to earn the qualification to the final tournament as the first leg was lost 3–1.
[265] Niculae is well known for his friendship with former Dinamo captain Cătălin Hîldan, who died on the pitch in a friendly match against Șantierul Naval Oltenița on 5 October 2000.