Mackellar resigned from his offices in August 1885, He was vice-president of the
Executive Council in the ministry of Sir
Patrick Jennings from 26 February to 23 December 1886, and then Secretary for Mines until the government was defeated on 19 January 1887. But though a good administrator, Mackellar was not a party man, and possibly for that reason did not hold parliamentary office again. In 1903 Mackellar was appointed a
Senator when
Richard O'Connor was made a Judge of the
High Court. Mackellar found, however, that he had too many interests in Sydney to be able to spare the time to attend the sittings which were then held at
Melbourne. He consequently opted not to stand for a full term at the
1903 federal election, and not long afterwards resumed his seat in the Legislative Council of New South Wales on 26 November 1903. Mackellar had been chosen as president of a
Royal Commission to investigate causes of the decline of the birth rate; he was largely responsible for the report that was issued. He had for some time been interested in the care of delinquent and mentally deficient children and in 1902 was appointed president of the state Children's Relief Department. He published this year as a pamphlet,
Parental Rights and Parental Responsibility, which was followed in 1907 by a thoughtful short treatise,
The Child, The Law, and the State, an account of the progress of reform of the laws affecting children in New South Wales, with recommendations for their amendment and more humane and effective application. In 1912 Mackellar visited Europe and the United States to study the methods of treatment of delinquent and neglected children, and issued a valuable report
Treatment of Neglected and Delinquent Children in Great Britain, Europe, and America on his return in 1913. He resigned his presidency of the state children's relief board in 1916. He still, however, retained his interest and in 1917 published an open letter to the Minister of Public Health on
The Mother, the Baby, and the State, and a pamphlet on Mental Deficiency, which shows his clear grasp of the subject was still apparent. ==Other interests==