We present new observations of the transiting exo-Neptune HAT-P-11b from a joint HST and warm Spitzer program to measure the transmission spectrum of its thick atmosphere. Our data cover a wide span of wavelength space, including warm Spitzer IRAC 3.6 & 4.5 micron photometry and Hubble WFC3 1.1-1.7 micron spectroscopy from our observations, as well as Kepler’s optical photometry centered at 632nm. Our WFC3 spectroscopic observations are among the first using HST’s new spatial scanning mode for optimised signal-to-noise spectroscopy. In addition, HAT-P-11 is one of the most active planet-hosting stars; observations of HAT-P-11b’s atmosphere therefore allow us to shed light on the role that stellar activity may play in shaping the atmospheric chemistry of Neptune-sized planets. We use the Kepler photometry to model and remove the effects of the stellar activity during and surrounding our warm Spitzer transit observations. Our combined observations provide constraints on the atmospheric chemistry at the day-night terminator and conclude in favor of a cloud-free atmosphere.