[6] He placed second in the club's best and fairest award that year despite being an underager, finishing behind only future-first-round AFL draft pick Clayton Oliver.

[21][22] Garthwaite was noted pre-draft for his skills in one-on-one contests, his strength in marking and spoiling and his ability to play on both small and tall forwards.

[27] After a pre-season of training plagued by groin soreness Garthwaite first donned the Richmond guernsey in a VFL practice match in March 2017 and remained with the reserves side for the remainder of the season.

[28][29] After an April win over Port Melbourne in which he was named in Richmond's bests, Garthwaite was noted by assistant coach Xavier Clarke as "a very good player at VFL level".

[34] He collected 10 disposals and four intercepts in the match, earning praise from senior coach Damien Hardwick who labelled his individual performance "terrific".

[37][38] He was again named as an emergency in round 13, but was called up to make his AFL debut when premiership defender David Astbury was ruled out with an ankle injury.

[42] He returned strongly to VFL level, but suffered a match-ending ankle injury in the first half of a match against Essendon's reserves side in early July.

[45] He paired well with All-Australian defender Alex Rance to blanket Essendon forward Cale Hooker early in that match before enduring a head collision with teammate Shaun Grigg that forced him to leave the field with a bloody face.

[48][49] Richmond's football manager Neil Balme publicly spoke out against the scheduling issue that forced both Garthwaite and teammate Sam Lloyd to miss the match, blaming a lack of cooperation between officials at the AFL and VFL.

[56][57] In the weeks immediately following the end of the 2018 season, Garthwaite underwent minor ankle surgery to fully repair damage done to the joint in his July 2018 injury.

[58][59] He returned to running exercises in December but developed Achilles tendinitis that saw him spend time in and out of the main pre-season training group through the end of February.

[62][63] In the same week that Garthwaite made a footballing return, his teammate and AFL backline leader Alex Rance suffered a season-ending ACL injury.

[79][80][81][82][83] Richmond coach Damien Hardwick acknowledged Garthwaite's struggles in the role, but said he had shown improvement week-to-week and labelled the experience important for his long-term development.

[57][95] Following the off-season retirement of fellow key-defender Alex Rance, Garthwaite was named by senior coach Damien Hardwick as one player in contention to replace him in the club's top-league side.

[98][99][100] Garthwaite was also scheduled to play in another reserves match the following week that was eventually cancelled due to safety concerns as a result of the rapid progression of the coronavirus pandemic into Australia.

[101] He could not earn selection to AFL level when the season began a week later, and was instead a non-playing emergency for the win over Carlton that was played without crowds in attendance due to public health prohibitions on large gatherings.

[111][112] He continued to play practice matches through June, being named as a non-playing emergency on multiple occasions but ultimately being overlooked for selection even despite strong performances and a long-term injury to AFL-tall-defender David Astbury.

[113][114][115] Garthwaite moved with the main playing group when the club was relocated to the Gold Coast in response to a virus outbreak in Melbourne in early-June, before suffering groin pain in July that saw him sit out practices matches through the early weeks of August.

[116][117][118] He made a brief return with one reserves game against Essendon in mid-August, but a flare up of the same soreness held him out again until mid-September, after which he remained with the unselected group into the beginning of the AFL finals series.