Thomas Gallagher was a single man, aged 23 and working as a labourer when he enlisted on 22 August, 1914. He was English by birth and gave as his next of kin his sister, Mrs. Agnes Bonet, living in Windsor, Ontario, Canada. He trained in Melbourne, was taken on strength with the 6th Battalion, D Company, and embarked for Egypt on board HMAT Hororata  A 20 on 19 October, 1914.

The 6th Battalion was amongst the first infantry units raised for the AIF. Like Thomas Gallagher, they were all from Victoria. After leaving Melbourne, the Hororata made its way to Albany, Western Australia, to join the ships from other states, before heading off to Egypt, arriviing on 2 December, 1914. After further training in Egypt, the 6th Battalion left for Gallipoli and took part in the second wave of the Anzac landing on 25 April, 1915.  Ten days later, the men were sent to capture the village of Krithia, where they suffered heavy casualties. The 6th Battalion took part in the retreat from Gallipoli to Egupt in December, then set sal for France in early 1916.

In the meantime, Thomas Gallagher was in Egypt, having been removed from the Gallipoli Peninsula on the HS Uysses and taken to Mena House on 23 August 1915. He was diagnosed as being deaf and with a fever, plus a varicocele - a swelling or lump on the testes. The fever was diagnosed on 25 August as influenza and he was moved to the No.2 General Hospital at Ghezirah.  He was released to the Anzac Base at Mudros and on 22 October, 1915,  was charged with being drunk in town on 16 October  - presumably at Mudros, where he had just been taken on strength. It was a minor offence - overdue by only one and a half hours, he was fined 2/6d. 

On 31 October, he was back with his unit, but again went AWL, more seriously this time. He was at Tel-el-Kebir and was absent from 22-25 January, 1916. For this he was punished with 21 Fiedl Patrol No.2. On 4 November, he returned to the Anzac Base at Mudros. It may have been because of continued health problems, because he was taken out of the 6th Battalion and transferred over to 1st Pioneer Battalion on 12 March, 1916, just at the time the 6th would have been heading off to France. Pioneer Battalions were attached to Brigades and were used for building duckboard paths and trenches, installing communications, mending roads and bridges and a multiude of other engineering tasks. Whether Thomas Gallagher was happy with the transfer is unknown, but on 5 April, 1916, he was charged with disobeying a lawful command of a superior officer and sentenced to 3 days Field Punishment No. 2. On 24 April, he was again charged with an offence, this time using obscene and abusive language towards a member of the Military Police.

He was transferred again on 3 July, 1916, and left the 1st Pioneers to join the Australian Mining Corps, but was declared supernumerary to the Establishment. He was back in hospital on 5 September, again with influenza, which he caught while on furlough in the UK. 

Thomas Gallagher was invalided out of the AIF on 5 November, 1917, discharged with deafness, Eoteresea and Lympane, and returned to Australia. On 5 February, 1918, he was finally dismissed form further service, presumably in Melbourne and was awarded a pension of 30 shillings per fortnight. He was then living at 86 Station St., Carlton. As Thomas Gallagher had no second name, was not married, and because his name was not unusual, it is impossible to trace hime any further.