Hornby was born in 1944 in Brandon, Manitoba, Canada, and was raised there and in London, Ontario.

Hornby received a Bachelor of Arts degree from the University of Western Ontario in 1965.

From 1965 to 1966, he was enrolled in the Near Eastern Languages and Literatures department of the Harvard University Graduate School of Arts and Sciences.

[2][5] Hornby engaged in the private practice of law at Perkins, Thompson, Hinckley & Keddy in Portland, Maine (1974–82; partner from 1975).

He held various officer and board positions including that of President (1977–79) at the Portland Society of Art (1975–84), which then operated both the Portland Museum of Art and the Portland School of Art (now Maine College of Art) during a period when the School attained national accreditation and the Museum embarked on its ambitious expansion program engaging Henry M. Cobb of Pei Cobb Freed & Partners to design the new Museum building with the support of Charles Shipman Payson.

[7] President George H. W. Bush appointed Hornby a United States District Judge in the District of Maine in 1990 to fill a seat made vacant by the appointment of Conrad Cyr to the First Circuit.

In 2009, Hornby received the 27th Annual Edward J. Devitt Distinguished Service to Justice Award.

[2][5][9][10][11] In 2014, Hornby received an Honorary Doctor of Laws degree and the Morton A. Brody Distinguished Judicial Service Award from Colby College, given to a judge "who embodies the qualities of integrity, compassion, humanity, and judicial craftsmanship exhibited by Judge Brody.

"[5][12][13][14] In April 2014, the Maine Legislature passed a resolution recognizing Hornby's achievement.

[3][5] He is a frequent contributor to the Green Bag, an "entertaining journal of law" dedicated to good legal writing.

[5] While the federal judiciary was seeking salary restoration, Hornby also wrote and spoke on that topic.

[17][18] The Green Bag has published "Beatitudes and Jeremiads"[19] as well as several chapters of Hornby's "Fables in Law: Legal Lessons from Field, Forest, and Glen," Aesopian legal fables for lawyers, judges, and law professors.

[20] Judicature has published three "imagined conversations" among fictitious former law school classmates now well along in their careers, on the topics of judicial opinion writing, the decline in federal civil trials, and public attention to federal judges.

Hornby uses his characters, including the federal trial lawyer Talagud Storey and the general counsel Manny G. Risk, to canvas the major issues surrounding these topics.

[22][23][24][25] For many years he updated pattern jury instructions for district courts within the First Circuit Court of Appeals (now updated by Judge Torresen)[3][26][27] and a case-based manual for opening statements and closing arguments in jury trials.

[3][5] When Hornby became a United States District Judge, Chief Justice Rehnquist appointed him to the Judicial Conference's Committee on Court Administration and Case Management (CACM) (1990-2000; chair 1997–2000) and as chair of the FJC's Committee on District Judge Education (1995–98).

In 2007, Chief Justice Roberts appointed Hornby chair of an ad hoc committee to secure federal judge salary restoration.

[29] Judge Hornby was elected to the Council of the American Law Institute in 1996 (ALI member since 1979), and took emeritus status in 2017.

He was an Adviser on the Restatement of the Law (Third), Restitution and Unjust Enrichment and Chair of the Awards Committee.

[30] Hornby was a Member of the National Academies' Standing Committee on Science, Technology, and Law (2006–13).

"Constitutional Limitations on Admissions Procedures and Standards" (with Ernest Gellhorn) 60 Virginia Law Review 975 (1974).