After postdoctoral fellowships with P. J. Bailey at the University of Edinburgh, Keith Izod at Newcastle University as the Wilfred Hall Research Fellow, and Polly Arnold at the University of Nottingham, Liddle began his independent academic career at the University of Nottingham with a Royal Society University Research Fellowship (2007–2015) held with a proleptic Lectureship. He was promoted to Associate Professor and Reader in 2010 and Professor of Inorganic Chemistry in 2013.
He moved to the University of Manchester in 2015 as Head of Inorganic Chemistry and Co-Director of the Centre for Radiochemistry Research. He held an Engineering and Physical Sciences Research Council Established Career Fellowship (2015–2021).
He was Chairman of COST Action CM1006, a 22 country, research network of over 120 research groups in f-block chemistry (2011–2015), is an advisor to the Commonwealth Scholarship Commission (2013–), and was an elected category 3 member of Senate, the University of Manchester (2016–2019).
Liddle's research is focused on synthetic inorganic chemistry, particularly making early transition metal, lanthanide, and actinide complexes to explore their structure, bonding, reactivity, and magnetism. In 2011 he reported a single-molecule magnet based on depleted uranium. In 2012 his research group was the first to synthesize a molecular terminal uranium(V) nitride.
In 2013 he reported the isolation of a terminal uranium(VI) nitride akin to examples previously restricted to cryogenic matrix isolation experiments. Also in 2013, his research group disclosed the first f-element cyclobutadienyl complexes.  In 2015 he reported a uranium(IV)-arsenido complex containing uranium-arsenic triple bonds. In 2019 his research group isolated a uranium(V)-dinitrogen complex. In 2021, Liddle reported a tri-thorium cluster featuring σ-aromatic actinide metal-metal bonding. Before the synthesis of this complex, examples of actinide-actinide bonding had been restricted to matrix isolation experiments and fullerene-encapsulated species.
In 2022, he reported a terminal neptunium(V)-mono(oxo) complex. Before the synthesis of this complex all transuranium-ligand multiple bond complexes required two or more metal-ligand multiple bonds to be stable.