We present far-infrared spectroscopy of supernova remnants (SNRs) using the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). We present the profiles of line and continuum spectra for an unpublished ISO sample of ~ 20 SNRs (or SNR candidates) in the Galaxy and Magellanic Clouds. Our ISO LWS sample includes SNRs that were not observed with Herschel spectroscopy, and the high spectral resolution of LWS reveals a number of far-infrared fine-structure lines. We used highly processed LWS data and further processed the data by carefully merging the spectra from different detectors to construct the ~ 40 - 200 micron band FIR continuum spectrum for each SNR. In most remnants, we detect one or more atomic fine-structure lines: [O I], [O III], [N II], and [C II]. In ~10 SNRs, we find evidence for line broadening with velocity dispersions of ~400-3000 km/s, indicating that they are from SN ejecta, some of which are the first evidence of high-velocity ejecta. We constrain the line emitting regions’ density and temperature from the two [O III] lines at 52 and 88 microns using the line excitation model. By applying the blackbody model fits to the continuum in a few SNRs in our sample, we estimate the lower limit of dust mass associated with SN ejecta. We present raster maps of 63 micron [O I] and 157 micron [C II] toward three SNRs and the LWS spectra of a few unpublished H II regions and compare them with our SNR sample in far-IR line intensity and ratios, and continuum properties. Our results will enable us to plan fruitful new observations with SOFIA and JWST, which is crucial to understand dust formation in SN ejecta and the early Universe.