This Letter reports the observation of Formula Presented production and a measurement of its cross section using Formula Presented of proton-proton collision data recorded at a center-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. The Formula Presented production cross section, with both the Formula Presented and Formula Presented bosons decaying leptonically, Formula Presented (Formula Presented, Formula Presented), is measured in a fiducial phase-space region defined such that the leptons and the photon have high transverse momentum and the photon is isolated. The cross section is found to be Formula Presented. The corresponding standard model predicted cross section calculated at next-to-leading order in perturbative quantum chromodynamics and at leading order in the electroweak coupling constant is Formula Presented. The observed significance of the Formula Presented signal is Formula Presented, compared with an expected significance of Formula Presented.