Upon completion of military duty in 1969, Judge McLachlan joined the Stamford office of Cummings & Lockwood and was an associate and partner of Cummings & Lockwood in various offices in Connecticut from 1969 to his appointment to the Superior Court in March of 1996, except from 1969 to 1973 when he was a principal in the Westport law firm of Berkowitz, Balbirer & McLachlan.

While in private practice, Judge McLachlan was active in Bar activities, including the Connecticut Bar Association, the American Bar Association and the American Academy of Matrimonial Lawyers. He served as Chair of the Connecticut Bar Association's Family Law Section, President of the Connecticut Chapter of the American Academy of Matrimonial Lawyers and chaired the ABA Section of Taxation's Domestic Relations Tax Committee and the ABA Family Law Section's Tax Committee. Judge McLachlan lectured on family law and family law tax issues. He authored or co-authored a number of publications, including the Connecticut Bar Journal's Annual Survey of Developments in Family Law for various years from 1976 through 1989, as well as a number of articles in the Family Advocate and other publications dealing with divorce taxation. Since becoming a judge, he has served on a number of commissions and task forces, including the Governor's Commission on Children, Custody & Divorce, the Chief Justice's Task Force on Pro Se Litigants and the Judicial Task Force on Pro Se Dissolutions in Connecticut and has been on the adjunct faculty of the University of Connecticut Law School. He presently represents the Judicial Branch on the Health Care Cost Containment Committee. Judge McLachlan is a fellow of the American Bar Foundation, a life fellow of the Connecticut Bar Foundation and continues his membership in the American Bar Association, Connecticut Bar Association and the Hartford County Bar Association. Judge McLachlan also is a member of the American Law Institute.

Judge McLachlan was appointed to the Appellate Court by Governor Rowland May of 2003.