The school's student body was 76.9% (500) White, 10.2% (66) Asian, 5.1% (33) two or more races, 4.9% (32) Hispanic, 2.8% (18) Black and 0.2% (1) American Indian / Alaska Native.

In 1925, the monks of Saint Mary's Abbey, then in Newark, purchased approximately 400 acres (160 ha) of Delbarton to use as a separate house for younger members for studying, finally settling on the property in 1927.

After some deliberation, Abbot Patrick O'Brien opened Delbarton School in 1939, appointing Father Augustine Wirth as the first headmaster.

Because of the fire, the school chose as its motto, "Succisa Virescit" (having been cut down, she grows back stronger), borrowing from the often destroyed Abbey of Monte Cassino in Italy.

Molestation allegations against a monk at Delbarton resulted in lawsuits being filed against the school by two former students who claimed to have been inappropriately touched by the Reverend.

[13] In 2018, the school acknowledged that 13 priests or monks, and one lay faculty member had abused 30 individuals over a period of decades, resulting in at least 8 legal settlements with seven other cases still unresolved at that point.

[14] In April 2021, Delbarton officially opened St. Benedict Hall, featuring a new library, Spada Commons, new office suite for guidance and college counseling and three new classrooms.

Over the past few years many respectable speakers have spoken to the student body including Dith Pran and Lech Wałęsa.

Delbarton students have also traveled to Spain on several organized summer trips and to Germany for World Youth Day with the school's religious educator David Hajduk.

Advanced math courses such as Linear Algebra, Multivariable Calculus, and AP Statistics are offered, as are independent study options.

[39] The 1993 team finished the season 11-0 after winning the Group III Parochial title with a 24-23 victory over Saint Joseph Regional High School.

The 2011 Delbarton soccer team won the Northwest Jersey Athletic Conference ("NJAC") regular season title as well as the NJSIAA Non-Public A North sectional championship and was ranked as high as #2 in the state.

The 2012 squad was undefeated/untied in winning the NJAC and also won both the inaugural Delbarton Invitational Tournament and the NJSIAA Non-Public A state championship, the latter with a 1-0 victory over the Pingry School on a last-minute goal.

Matt Clausen (MSG Varsity NJ POY) and Greg Siefert (NSCAA Regional All-American) were both 2012 Star-Ledger first team All-State selections.

In 2014, Delbarton once again took home the Non-Public A state championship, its third in a row, with a 3-0 win in the tournament final against Notre Dame High School.

[49] In 2015, the soccer team won its fourth consecutive Non-Public A title with a 2-0 win against Christian Brothers Academy and finished the season top-ranked in the state by NJ.com.

[55] In 2023, the team repeated as champion and won its eighth MCT title with a 58-36 win against Madison High School in the finals.

Beginning in 2008, Delbarton, led by head coach Bruce Shatel, won six straight NJSIAA Non-Public State Titles: 2008, 2009, 2010, 2011, 2012, and 2013.

[61] In 2013, Delbarton defeated Don Bosco Preparatory High School 4-0 at the Prudential Center in Newark in the Non-Public State Final.

The Green Wave also defeated Don Bosco Prep in the Gordon Cup Final 2-1 at Codey Arena in West Orange, New Jersey.

Delbarton defeated St. Augustine Preparatory School by 54-7 in the Non-Public A championship, bringing home a state title for the first time since 2011.

Joe Tavoso '15, Ty Agaisse '16, and Patrick Glory '18 all won individual state championships, all three later wrestling at Princeton.

Under the direction of Kent Manno, Delbarton was crowned champions of the Morris County Tournament for an unprecedented 10 consecutive years, from 1983 to 1993.

[79] The team won its fifth Morris County Tournament title in 2017, defeating Randolph High School by a 6-5 score in extra innings in the final.

[82] Under Coach Bruce Shatel, Delbarton won the Non-Public A state championship in 2017 with a 2-1 victory in the final of the tournament against St. Augustine Preparatory School.

[96] Delbarton returned to the top of New Jersey lacrosse after eight years in 2014, capturing its fourth TOC state title with a win over Summit High School by a final score of 7-5.

[109] On April 23, 2010, at the 116th Penn Relays, Pat Schellberg won the high school boys' one mile (1.6 km) run championship and broke the track meet's event record.

[111] The club went undefeated throughout the 2015 season, outscoring its opponents 246-32, and defeating St. Augustine Preparatory School by a score of 33-21 in the single-school state championship.

Environmentalists and several Morris Township residents had been working against the project that would develop 71 acres (290,000 m2) of zoned land adjacent to the Jockey Hollow Park.

[116] In 2012, various media outlets reported news of sexual misconduct by several monks from Saint Mary's Abbey who also worked at Delbarton School.