After training with the club, it was revealed that they wanted to sign him due to his good performances for Huachipato and his nomination for the 2004 Copa América in Peru.

However, his move to Germany failed to materialise and he remained with Huachipato where his performance improved considerably, scoring 14 goals in 39 games.

In July 2006, after a very good Torneo de Apertura, Millar was nearly transferred to CD Tenerife and was also linked with FC Terek Grozny, but later rejected the offer for personal reasons.

Once he recovered, he made his debut on 20 March 2007 against the Venezuelan side Caracas FC at the Copa Libertadores and scored his club's first goal in a 4–0 win over LDU Quito.

After a good start to the season, his form declined due to off field problems, and he ended up being relegated to the substitutes bench.

He attempted to return to Chile, but was shunned by Colo-Colo's coach Claudio Borghi and shortly after loaned to Once Caldas.

After his loan with Once Caldas had finished, he returned to Colo-Colo in June 2008, despite Fernando Astengo not planning on featuring him in the team,.

During the 2010 season he performed consistently well with Colo-Colo, and was selected for Chile's FIFA World Cup 2010 team and voted the Chilean Footballer of the Year in the Anfp awards.

[8] After not being included in the 23-man World Cup squad, Millar returned to play for Chile in friendlies against Mexico and Haiti.