Muižnieks had solid career as player, which is followed by prolific coaching career. As head coach Muižnieks has led his teams to multiple championships, winning titles in domestic and international competitions. So far he has been most successful in Latvia, where he has guided his teams to six Latvian League titles (2002, 2003, 2004, 2005, 2006, 2008). First success in European club tournaments Muižnieks had with BK Ventspils, where in 2002/2003 his squad he finished third in FIBA Europe Champions Cup, where in bronze medal game they upset KK Hemofarm and their up-and-coming star Darko Milicic. Later, in 2004/2005, Muižnieks led Ventspils to ULEB Cup quarterfinals in 2004/2005. Few years later in 2007/2008, his Barons/LMT won FIBA EuroCup title.
In 2011/2012 as coach of Polish side Trefl Sopot he was named as the Coach of the Year in Poland. Under coach Muižnieks Trefl Sopot won Polish Cup and for the first time made league finals, where they lost only in Game 7 to Asseco Prokom. Success under Muižnieks in Polish league clinched EuroCup berth for Trefl Sopot. In summer of 2012 Muižnieks left Trefl Sopot because of unpaid salaries.
Muižnieks was also Latvian National Team head coach from 2004 to 2008.
- 1986-1988: BK VEF Rīga
- 1988-1989: PBC CSKA Moscow
- 1989:1990: BK VEF Rīga
- 1990: Erie Wave
- 1990-1993: ASK/Brocēni/LMT
- 1993-1994: Maccabi Haifa B.C.
- 1994-1995: ASK/Brocēni/LMT
- 1996-1997: Shakhtar Donetsk
- 1997: Liepāja Baltika
- 1997-1999: BK Ventspils
- 1999-2001: BK Ventspils (assistant)
- 2001-2007: BK Ventspils
- 2007-2008: Barons/LMT
- 2009-2012: Trefl Sopot
- 2012-2013: Kryvbasbasket
- 2013-present: BC Khimik
- "KARLIS MUIZNIEKS basketball profile". eurobasket.com. Retrieved 2010-04-02.
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