He has served as Deputy District Attorney, 1968-1974, and Senior Trial Deputy District Attorney, 1974-1976, Alameda County District Attorney's Office; Executive Director, California District Attorneys Association, 1976-1979; Special Assistant Attorney General, 1979-1980, and Senior Assistant Attorney General, California Department of Justice, 1980-1983; Deputy Director, Special Projects for the Governor, Office of Planning and Research, State of California, 1983-1984; Founding Director and Chief Counsel, National School Safety Center, a partnership of Pepperdine University and the United States Departments of Justice and Education, 1984-1986; and Adjunct Professor, Graduate School of Education and Psychology, Pepperine University, 1985-1986. Justice Nicholson has been Judge for the Sacramento Municipal Court, appointed in 1987 and elected 1988; Judge, Superior Court, County of Sacramento, appointed in 1989; nominated to the Court of Appeal, Third Appellate District, as Associate Justice in 1990 and subsequently confirmed by the Commission on Judicial Appointments; and served as an appellate justice from 1990 until 2018, when he retired.

He served on the judiciary for 32 years in all, the final 28 years on the Court of Appeal, Third Appellate District. Voters in his 23-county appellate district re-confirmed him three times, in 1994, 1998, and 2010. While there, he participated in almost 30 percent of the Third Appellate District’s caseload; was directly involved in 10,586 cases, and of them, authored 3,472 opinions, and concurred in 7,114 opinions. More than 300 of his opinions were published. He served as a justice pro tem seven times on the California Supreme Court in 2018, 2014, 2009, 2009, 2003, 2002, and 1992.

He has been Statewide Co-Chair and Principal Author, Proposition 8, Victims' Bill of Rights, adopted by voters in 1982; Member, Victims of Crime Committee, American Bar Association, 1982; Member, Planning Committee, National Judicial Conference on the Rights of Victims of Crime, 1983; Chair, Court Technology Committee, Sacramento Municipal Court, 1987-1989; Member, Executive Committee, Sacramento Municipal Court, 1988; Member, Board of Trustees, Law Library, County of Sacramento, 1988-1989; Member, Commission on the Future of the California Courts, 1991-1993; Member of the Commission's Executive Committee and Chair of the Commission's Appellate Courts and Technology Committees; Member, Select Committee of Advisors, Pacific Law Journal, McGeorge School of Law, 1992-1996; Member, Forum on Justice & Public Safety in the 21st Century, 1994-1998; Master of the Bench, Anthony M. Kennedy American Inn of Court, 1995-1998; Member, Model Courtroom Planning and Design Advisory Committee, National Judicial College, 1997; and Chair, Planning Committee, Symposium on Shaping the Future of Legal Information, Bernard E. Witkin State Law Library, 1998.

His many awards and honors have included Outstanding Contribution to the Criminal Justice System, 1975, Dedicated Work in the Field of Criminal Justice Legislation, 1976, and Outstanding Individual within Criminal Justice System, 1981, all three by Citizens for Law & Order; Distinguished and Dedicated Service, California District Attorneys Association, 1980; President-Elect's Victims of Crime Task Force, 1981; Member of National Citizens Crime Prevention Campaign, 1982; Dean Gordon Schaber, Victims of Crime Resource Center, McGeorge School of Law, 1983; Honorary Member, American Association of Law Libraries, 1997, there are only three honorary members in this 5,000 member organization; Recipient, Award for 10 years of of support and friendship for the goals and purposes of UNITY, a joint project of the Asian, La Raza, and Wiley Manuel Bar Associations, 1997; Recipient, Government Technology Leadership Award, May 1998; and Recipient, Award of Merit for Outstanding Contributions to the Judiciary, American Judges Association, August 1998, prior recipients include United States Supreme Court Justice Sandra Day O'Connor, Chief Justice Richard Holmes of Kansas, and Dean Robert Payant, National Judicial College.

Justice Nicholson has also been Editor-in-Chief, The Crime Victims' Handbook, California Department of Justice, 1981; Adjunct Professor, Graduate School of Education and Psychology, Pepperdine University, 1985-1986; Faculty, National Conference on Juvenile Justice, 1986; Faculty, National Judicial College, 1986, 1990; Co-author, School Crime and Violence: Victims' Rights, 2d edition, Pepperdine University Press, 1992; Author, Victims' Rights, Remedies, and Resources: A Maturing Presence in American Jurisprudence, 23 Pacific Law Journal 815, 1992; Co-author, Retooling Criminal Justice: Forging Workable Governance from Dispersed Powers, Selected Essays, The National Conference on Legal Information Issues, American Association of Law Libraries, 1996; Faculty, Advanced Criminal Law Seminar, Center for Judicial Education and Research, 1997; and Faculty, Appellate Courts Institute, Center for Judicial Education and Research, 1993-1998.